The present invention relates in general to demodulators and, more particularly, to an FM (frequency modulation) detector circuit with means to substantially eliminate the harmonic distortion in the demodulated output signal.
FM demodulators are used in many consumer electronics, i.e., stereos and cellular telephones, to recover the information encoded in an FM transmission. One FM demodulator, as disclosed in U.S. Pat. No. 4,375,618 issued Mar. 1, 1983 to Jett, Jr., includes a gain controllable multiplier and a quadrature phase shift network which are responsive to an applied FM signal. The phase shift network is typically tuned to provide .pi./2 radians of phase lag to the FM signal at the center (carrier) frequency. The phase shifted FM signal is also applied to the multiplier for providing an output signal the amplitude of which is proportional to the deviation of the frequency of the FM signal from the tuned quadrature frequency of the phase shift network. Thus, the FM input signal has been converted to a more directly useful amplitude modulated (AM) signal capable of driving output devices such as speakers and headsets.
One problem associated with the aforedescribed FM demodulator is that the output signal contains odd harmonic distortion attributed to the nonlinear frequency behavior of the passive components of the phase shift network. In addition, second harmonic distortion is introduced by drift in the tuned quadrature frequency and delays within the multiplier. The harmonic distortion may attenuate the output signal leading to possible misinterpretation of the FM signal. To solve the distortion problem, the '618 patent uses a complex feedback circuit comprising an audio stage and a fullwave rectifier which are coupled between the output of the multiplier and the control input of a current source. The output of the current source is coupled to the multiplier for providing a gain control signal to adjust its linear range. The audio stage extracts the second harmonic from the output signal and combines it with the fundamental in the fullwave rectifier to generate a third harmonic which modulates the gain control signal and removes the third harmonic component of the output signal. The circuit works fine to remove that portion of the distortion attributed to the third harmonic; however, it may have limited effectiveness with higher order harmonics and may not compensate for second harmonic distortion.
Another method to remove the harmonic distortion is to use a double-tuned circuit comprising a first tuned coil with mutual coupling to a second tuned coil. Each tuned coil will have a different center frequency which increases the linear bandwidth of the phase shift network. The double tuned circuit has proven to be difficult to tune in production and sensitive to component variation and drift.
Hence, there is a need for a simple FM detector circuit having means to substantially eliminate the distortion attributed to the total harmonic spectrum of the output signal such that the amplitude thereof is truly proportional to the deviation of the instantaneous frequency of the FM signal from the center frequency.